As electric and electronic equipment has been miniaturized and integrated at a higher level, intense interest has recently been shown towards the problem of reducing the heat generated by various devices, e.g., power transistors, thyristors, rectifiers, transformers, etc., set in such equipment.
In using various heat-generating devices as cited above, heat reduction has so far been carried out by attaching a heat sink thereto or mounting them on a metallic chassis. In addition, the devices as described above have been mounted on a heat sink or chassis via an electrical insulating material, because it is generally undesirable to bring a heat reducing material into direct contact with electronic or electric devices from the standpoint of designing various types of equipment and in view of safety.
As for the electrical insulating material described above, there have so far been known organic rubbers, plastics and the like as organic types, or porcelain, glass fibers, mica and the like as inorganic types.
However, the organic insulating materials as cited above are poor in heat conductivity although they have satisfactory electrical insulating properties. The inorganic insulating materials, on the other hand, are satisfactory with respect to the heat conductivity, but not only are they poor in workability, because of their hardness and brittleness, but also it is difficult for them to come into close contact with the surface of a heat-generating material as well as the surface of a heat-reducing material since their surfaces are not of the proper condition. All of those materials, therefore, are not suitable for an electrical insulating material for which both high heat conductivity and excellent electrical insulating properties are required.
In recent years, metal oxides such as aluminum oxide and metal nitrides such as boron nitride and aluminum nitride have been used as inorganic fillers, resulting in an improvement in heat conductivity.
However, some extent of heat conductivity can be brought about by aluminum oxide, but it is still insufficient; while boron nitride and aluminum nitride have a drawback of high price, they do have good performance.
In addition, the heat conductive silicone composition containing the magnesium oxide obtained by burning a specified magnesium hydroxide at a temperature of 1,100.degree.-1,600.degree. C. is disclosed in Japanese Tokkai Hei 5-239358 (the term "Tokkai" as used herein means an "unexamined published patent application"). When the magnesium oxide is used as the filler for imparting heat conductivity to a silicone rubber, however, there occurs a phenomenon in that the silicone rubber molding crumbles into decay through the cracking of the silicone rubber under a high temperature-high humidity condition. One main reason for this is that the magnesium oxide has hygroscopicity and it gives strong alkalinity as a result of moisture absorption.
Such being the case, we have made intensive studies for obviating the above-described defect of the magnesium oxide. Thus, it has been found that a satisfactory result can be obtained by rendering the surface of magnesium oxide hydrophobic, thereby achieving the present invention.